In the smallest realms of the universe, at the subatomic level, the laws of relativity do not apply. Instead, the strange world of quantum physics rules. At the heart of quantum physics is a concept known as the uncertainty principle, a rule that maintains that it is impossible to know both the position and the momentum of a subatomic particle. You could say that all this principle is really saying is that it’s impossible to know these things. Clearly the universe is not so strange that a subatomic particle doesn’t actually have both a position and a momentum, right?
Then again, the subatomic universe is up to some pretty strange things.
This has been famously demonstrated in an experiment known as the double slit experiment.
In these experiments, photons or electrons are emitted toward a barrier with one or more slit openings in it. On the other side of the barrier is a phosphorescent screen that records the impact of each electron with a bright dot. If a beam of electrons is aimed at a barrier in which there is one slit carved, the wave will propagate through the slit in a predictable fashion, much like waves of water through a small gap in a wall, and will create an expected image on the screen opposite. (Fig. 1)
If a beam of electrons is aimed at a barrier in where there are two slits carved out, the wave spreads out as before, but this time, because there are two points of entry, two waves will be produced. The two waves will magnify each other where two crests intersect and diminish each other where two troughs come together. This creates what is known as an interference pattern and that pattern will be reflected in the image that results. (Fig. 2)
Now, if instead of a beam of light or particles, single electrons or photons are fired one by one at a barrier in which there is one slit carved, a small band will appear on the plate opposite where the electrons have struck. The narrower the slit, the narrower the width of the band of electrons captured. (Fig. 3)
However, if single electrons are fired one by one at a barrier in where there are two slits, a surprising thing happens. If the electrons are observed—that is, if light is shined upon them—they behave as you would expect them to and produce two bands on the photographic plate. (Fig. 4)
However, if the electrons are unobserved, instead of seeing two bands on the plate opposite as one would expect, we get an interference pattern as we did with the plane wave of light. (Fig. 5)
But how is this possible? The electrons are coming through one at a time. There is no reason for there to be an interference pattern—what are they interfering with? The surprising answer is that the electrons (or any subatomic particles, really) are behaving simultaneously like particles and waves, and are interfering with themselves. That is, each particle is behaving as if it went through both slits at the same time and interfered with itself.
Physicist Richard Feynman proposed that the particles are not simply behaving as if they had gone through both slits, but that the electron actually does go through both slits. Feynman argued that in traveling from the source to the phosphorescent plate, each individual particle actually traverses every possible trajectory simultaneously—perhaps even through distant points in the universe—on its way to the plate. In the aggregate, the paths this electron takes yield the same result as a wave-function.1
What this means is that the location of such an electron can only be stated as a function of probability, not certainty. That’s a disquieting notion. Disquieting enough to provoke Einstein to make his famous comment objecting to quantum physics saying, “God does not play dice (with the universe)!” We tend to think of scientists as those who can provide us with the answers. These are, after all, the people in the white lab coats with the expensive microscopes and all the computers and equations. And the best they can do when asked for the position of a subatomic particle is that it’s probably over there. Probably?
In the end, quantum physics does not predict a single definite result for an observation; it predicts a number of different possible outcomes and the likelihood of each. So, if you were looking to science for certainty and fixed answers, quantum mechanics suggests you were looking in the wrong place.
1 Greene (2000), 110-111